* Reject unknown imported cities from queries

This commit fixes a bug where the city report returned `N/A` entries.
The functions that build imported data queries were using SQL
`COALESCE`, assuming city data is `NULL` when unknown, when actually its
unknown value is `0`.

This commit addresses the problem using SQL `NULLIF` combined with the
previous `COALESCE` call. With this change both `0` and `NULL` are
treated as unknown.

Since 1cb07efe6dd800c906110858e55a6788cc2cdccf cities can be `NULL`, but
previously we saved `0` as unknown.

Closes #1960

* Add entry to CHANGELOG

* Ignore cyclomatic complexity Credo check